This invention relates to a filter element for a liquid filter of the type employing a filter medium, such as a fabric bag or sleeve or other porous panel or wall through which liquid is to pass upon filtration, and wherein a filtration aid such as diatomaceous earth is used in combination with the filter medium to form a layer on the outside of the filter medium. Dirt then becomes deposited on the layer of filtration aid rather than directly on the filter medium.
More particularly the invention relates to a filter element of the type comprising a support frame and a fabric bag or sleeve covering in the frame as well as to the frame itself.
Filters which employ a filter medium and a filtration aid in order to avoid premature blinding of the medium and thereby to extend the cycle time between back washing procedures, generally operate on the principle that when the filter is turned off, the filter aid and dirt collected thereby fall to the bottom of the container. When the filter is started up again, inlet fluid serves to stir up the filtration aid usually together with at least some dirt from the bottom of the filter container so that the filter aid becomes coated on to the filter medium once more.
During this process the filtration aid and dirt in the bottom of the container inflict wear on the filter medium, namely the fabric of the bag or sleeve. The severity of the wear on the bag or sleeve depends largely on the direction and velocity of the flow of incoming liquid which stirs up the mixture of dirt and filtration aid. It has now been found that, depending on the exact configuration, an inlet arrangement such as that described in my granted South African patent No 93/0761 can lead to undue wear on the bag or sleeve.
More particularly, the type of filter described in my said earlier South African Patent utilizes filter elements which are of an elongate construction and have a plastics injection moulded frame having, in cross-section, a number of longitudinally extending ribs radiating from a central axis and the outer edges of which support the fabric sleeve. The construction of the frame is dictated, to some extent, by the tool design and, as a consequence, the operatively lower end of each frame was heretofore moulded as a ring encircling the outer edges of the ribs. The end of the frame was left open in order to allow full fluid flow through the end of the sleeve.
This construction of the lower end of the frame is now believed by applicant to be at least partly responsible for excessive wear on the lower end of the sleeve which is experienced at least in some instances.
It is accordingly an object of this invention to provide a filter element and a frame therefor which is better adapted to diminish or accommodate this abrasive effect on the fabric at the lower ends of the bags or sleeves which are in the immediate vicinity of the settled dirt and filtration aid when it is stirred up.
In accordance with this invention there is provided a liquid filter element frame comprising an elongate injection moulded body having a plurality of support ribs radiating from a central axis thereof and a support ring encircling the outer edges of the ribs at the operatively lower end thereof, the frame being characterized in that the ring has attached thereto an end disc having a smoothly contoured edge for supporting the lower end of a sleeve of filter fabric, in use.
Further features of the invention provides for the disc to have perforations coinciding with the spaces between adjacent ribs to provide for fluid flow through the disc; and for the elongate moulding to have support rings encircling the ribs at spaced positions along the length thereof.
The invention also provides a sleeve made of filter fabric and adapted to be used as a cover for the element frame defined above wherein the operatively lower end of the sleeve is made of a double layer of filter fabric.
A further feature of this aspect of the invention provides for the operatively upper end of the sleeve of filter fabric to also be made as a double layer of fabric. The purpose of the latter is to protect that region of the bag which is located uppermost and which is maintained in position by means of an elastomeric band, so that it does not become worn before the normal life of the sleeve has come to end.
In order that the invention may be more fully understood one embodiment of thereof will now be described with reference to the accompanying drawings.